Carburized and quenched member and method for production thereof

ABSTRACT

A carburizing and hardening method enhances strength while sufficiently reducing hardening strain, without increasing the production cost, and a carburized and hardened member produced thereby. The raw material is an alloy steel which contains Fe as a main component, 0.10 to 0.50 wt. % C and 0.50 to 1.50 wt. % Si and having a hardenability J, based on an end quenching test, in a range of 35 to 50 (at 12.5 mm). After the raw material is formed into the desired shape, a carburized layer is formed by carburizing in an oxidation inhibiting atmosphere. After the carburizing, quenching is performed with cooling, uninterrupted by temperature rise, from a pearlite transformation point (A1 point) to a martensite transformation start point (Ms point), and with a severity of quenching H in a range of 0.01 to 0.08 (cm −1 ).

TECHNICAL FIELD

The present invention relates to a carburized and hardened member thatis excellent in fatigue strength and dimensional accuracy, and aproduction method for the member.

BACKGROUND ART

For example, for power transmission component parts of an automatictransmission, for example, gears and the like, carburized and hardenedmembers subjected to a carburizing and quenching process are often usedin order to increase the surface hardness and the toughness.

Conventional carburized and hardened members are normally produced byforming a case hardening steel (JIS: SCM420H, SCR420H, SNCM220) or thelike into a desired shape, and then gas-carburizing the steel in acarburizing atmosphere, and then quenching it in an oil or the like.

As for the carburized and hardened members, cost cut and performanceimprovement are demanded more strongly than ever.

In order to achieve both a cost cut and a performance improvement, it isnecessary to remove each of problems of the conventional carburized andhardened members produced from a conventional case hardening steel by anordinary carburizing and quenching method.

One of goals regarding the carburized and hardened members is to furtherimprove the post-carburizing and quenching process strength and, at thesame time, improve the dimensional accuracy by reducing or suppressingthe hardening strain.

However, improved hardenability normally leads to increased hardeningstrain, as well known. There is a possibility that the strength prior tothe carburizing and quenching process may increase resulting in degradedprocessability and therefore increased cost of processing.

The present invention has been accomplished in view of theaforementioned problems of the conventional art. It is an object of thepresent invention to provide a carburized and hardened member thatallows strength enhancement while sufficiently reducing the hardeningstrain, and a production method for the carburized and hardened member.

DISCLOSURE OF THE INVENTION

A first aspect of the present invention is a carburized and hardenedmember production method characterized in: that an alloy steel whichcontains Fe as a main component and contains 0.10 to 0.50 wt. % of C and0.50 to 1.50 wt. % of Si and whose hardenability J based on an endquenching test is in a range of 35 to 50 (at 12.5 mm) is used as a rawmaterial; and that after the material is formed into a member of adesired shape, a carburized layer is formed by performing a carburizingprocess in an oxidation inhibitive atmosphere; and that after thecarburizing process, a quenching process is performed in such acondition that cooling is monotonously performed from a pearlitetransformation point (A1 point) to a martensite transformation startpoint (Ms point), and such a condition that a severity of quenching H isin a range of 0.01 to 0.08 (cm⁻¹).

The aforementioned hardenability J based on an end quenching test is avalue acquired by an end quenching test method prescribed in JIS: G0561(generally termed “Jominy end quench test method”). Furthermore, theindication of (at 12.5 mm) means that the value of hardenability J is avalue of hardenability J regarding a position of 12.5 mm from the watercool-side end surface of a rod-like test piece in the Jominy end quenchtest method.

The aforementioned severity of quenching H is a widely used indexespoused by Grossmann et al. to indicate the strength of quenching, andis defined as in H=0.5×(α/γ) where γ is the heat conductivity (kcal/mh°C.) of a steel to be processed, and α is a surface heat transfer factor(kcal/mh²° C.) of the steel in a hardening atmosphere.

In the present invention, a specific alloy of which the C content andthe Si content and the hardenability J are within the specific ranges isused as a raw material. After a carburized layer is formed by performingthe carburizing process in the oxidation inhibitive atmosphere, thequenching process is performed so as to fulfill the aforementionedconditions of monotonous cooling and the aforementioned condition ofspecific severity of quenching H. That is, only after the materialcharacteristics and the production conditions are fulfilled, it becomespossible to provide a carburized and hardened member in which thestrength is enhanced while the hardening strain is sufficiently reduced.

This will be further explained. The setting of the C content within therange of 0.1 to 0.50 wt. % makes it possible to secure an appropriatetoughness and an appropriate strength of a non-carburized portion(internal portion) after the carburizing and quenching process. If the Ccontent is less than 0.1 wt. %, the aforementioned effect is notsufficient. If the C content exceeds 0.50 wt. %, the pre-quenchinghardness becomes excessively high, thus creating a possibility ofincreased processing cost and reduced toughness. Furthermore, due toincreased structural transformation rate of the interior of thenon-carburized portion following the carburizing and quenching process,transformation stress increases, and due to great quenching strain, thecomponent part accuracy may degrade.

Furthermore, in the present invention, the member positively contains Sias a component, and the content thereof is 0.50 to 1.50 wt. %. Thecarburizing process is performed in an oxidation inhibitive atmosphere.Therefore, it becomes possible to achieve improved plane fatiguestrength, improved hardenability, improved resistance to tempersoftening, etc, while reducing the intergranular oxidation, which islikely to occur at the time of the carburizing process.

If the Si content is less than 0.50 wt. %, the aforementionedimprovement effect is small; in particular, there is a problem ofreduction of intergranular oxidation preventative effect at the time ofthe carburizing process. Conversely, if the Si content is greater than1.50 wt. %, the improvement effect becomes saturated, and uniformaustenitization prior to quenching is difficult. In order to prevent orcurb degradations in the plastic processability, the cuttingprocessability and the formability of the material, it is preferablethat the Si content be less than or equal to 0.70 wt. %. Therefore, apreferable range of the Si content is a range greater than 0.50 wt. %and less than or equal to 0.70 wt. %.

The hardenability J of the material is limited within the range of 35 to50 (at 12.5 mm). Therefore, excellent hardening effect can be achievedeven if the range of the severity of quenching H is limited to theaforementioned range. If the hardenability J is less than 35, it becomesimpossible to achieve sufficient hardening effect on the carburizedlayer and the non-carburized portion (internal portion) in the quenchingprocess following the carburizing process, and it is thereforeimpossible to achieve a desired strength enhancement. Therefore, it ispreferable that the hardenability J be greater than or equal to 38. Ifthe hardenability J exceeds 50, the structural transformation rate ofthe internal portion, that is, the non-carburized portion, rises, sothat the transformation stress increases and the hardening strainbecomes more likely. If the hardenability J is higher, the hardnessprior to the carburizing and quenching process is correspondinglyhigher, so that processability, such as the plastic processability priorto the carburizing process, the cutting processability, etc., degrades.Therefore, in order to prevent such degradation of workability, it ispreferable that hardenability J be less than or equal to 45.

The severity of quenching H is limited within the range of 0.01 to 0.08(cm⁻¹). If the alloy having the specific amount of carbon and having thehardenability is used, it becomes possible to substantially prevent orreduce the hardenability strain at the time of hardening process andtherefore secure excellent dimensional accuracy.

If the severity of quenching H is less than 0.01 (cm⁻¹), it isimpossible to achieve sufficient hardening effect on the carburizedlayer and the non-carburized portion (internal portion) in a hardeningprocess following the carburizing process as in the case where thehardenability J is less than 35. Therefore, desired strength enhancementcannot be accomplished. If the severity of quenching H is greater than0.08 (cm⁻¹), the transformation stress increases due to, particularly,increased structural transformation rate of the internal portion, thatis, the non-carburized portion, and therefore the hardening strain islikely to occur, as in the case where the hardenability J is greaterthan 50.

The quenching process is performed under the condition that the coolingmonotonously occurs from the A1 point to the Ms point, in addition tothe condition of the range of severity of quenching H. The term“monotonously” herein means that re-heating is not performed during thecooling process, that is, there is no rise of the material temperatureduring the cooling. Therefore, examples of the case where the conditionof monotonous cooling is fulfilled include a case where the materialtemperature continues to fall, and a case where if the temperature stopsfalling during the process, the temperature remains constant and neverrises, and then starts falling again. Furthermore, changes in thecooling rate are allowable.

As the monotonous cooling is adopted as an essential condition,precipitation of carbides can be substantially prevented or reduced.

With regard to the monotonous cooling condition, it is possible toselect a cooling condition such that the cooling does not enter a regionof a nose of an S curve indicated in an isothermal transformationdiagram within the carburized portion. This selection secures sufficientmartensite transformation.

Although this may be a repeated statement, the present inventionprovides a carburized and hardened member in which the strength isenhanced while the hardening strain is sufficiently reduced, as theinvention comprises the aforementioned C content, the Si content, thehardenability J, the carburizing process in an oxidation inhibitiveatmosphere, and the quenching process that fulfills the condition of themonotonous cooling and the condition of the specific severity ofquenching H. If any one of these elements is absent, the intended objectcannot be achieved. The present inventors have discovered this throughmany experiments.

A second aspect of the present invention is a carburized and hardenedmember produced by the above-described production method, characterizedin that a surface hardness of the carburized layer is in a range of 700to 900 Hv, and an internal hardness of a non-carburized portion locatedinward of the carburized layer is in a range of 250 to 450 Hv.

This carburized and hardened member is produced by adopting theabove-described production method and by adjusting the component rangeprocessing condition so as to restrict the surface hardness of thecarburized layer and the internal hardness of the non-carburized portionwithin the aforementioned ranges. Therefore, it becomes possible tosecure a static strength (tensile strength, flexural strength, torsionalstrength, etc.) and a dynamic strength (plane fatigue strength, bendingfatigue strength, torsion fatigue strength, etc.) in a region from thesurface to the internal portion (core portion), with respect to thedistribution of stress applied to the member which results from theoperating stress caused on the member by load applied to the member andthe stress concentrated adjacent to the surface of the member due tobumps and dips, holes, etc. of the member.

If the surface hardness of the carburized layer is less than 700 Hv, aconceivable problem is that strength cannot be secure corresponding tothe stress concentration adjacent to the surfaces of the member. Anotherconceivable problem is insufficient abrasion resistance in outermostsurface. If the surface hardness is greater than 900 Hv, production ofcarbide, such as cementite and the like, in the surface layer isconceivable. Therefore, a conceivable problem is insufficient strengthand, more particularly, reduced toughness.

If the internal hardness of the non-carburized portion is less than 250Hv, the problem of insufficient strength and, more particularly,insufficient static strength, can be considered. If the internalhardness is greater than 450 Hv, the following problem is possible,taking the rate of transformation of structure into consideration. Thatis, when a hardening process is performed so as to secure 450 Hv, agreat transformation stress occurs, which causes a great hardeningstrain and therefore makes a factor of degradation in component partsaccuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a rotating bending fatigue test piece.

FIG. 2 a is a plan view of a toothed gear for evaluation.

FIG. 2 b is a sectional view the toothed gear for evaluation.

BEST MODES FOR CARRYING OUT THE INVENTION

In the production method for a carburized and hardened member accordingto the first aspect of the present invention, it is preferable that thecarburizing process be performed in a reduced-pressure atmosphere havinga reduced pressure of 1 to 30 hPa. Therefore, it becomes possible toeasily provide the oxidation inhibitive atmosphere through pressurereduction, and therefore sufficiently prevent intergranular oxidation atthe time of carburization. The value of the reduced pressure of thereduced-pressure atmosphere being less than 1 hPa is excessive forsubstantial prevention of oxidation. If such value of the reducedpressure is required, the device for the pressure reduction needs tohave high capability for pressure reduction, and creates a problem ofcost increase. If the value of the reduced pressure is higher than 30hPa, the oxidation preventing effect degrades, and furthermore, otherproblems, such as production of soot in the carburizing furnace, and thelike, occur.

It is also preferable that the carburizing process be performed in anatmosphere containing an inert gas as a main component. This also makesit possible to easily form the oxidation inhibitive atmosphere. Examplesof the inert gas include nitrogen gas, argon gas, etc.

It is also preferable that the carburizing process be performed so thata surface carbon amount in the carburized layer becomes 0.6 to 1.5 wt. %(claim 4). The surface carbon concentration in the carburized layeraffects the surface hardness of the carburized and hardened member. Ifthe surface carbon amount in the carburized layer is less than 0.6 wt.%, there occurs a problem of insufficient surface hardness. If thesurface carbon amount is greater than 1.5 wt. %, the precipitation ofcarbide becomes great so that the hardenability of the base remarkablydegrades and the surface hardness becomes insufficient.

It is also preferable that intergranular oxidation progressing from asurface of the raw material be at most 3 μm. That is, it is preferableto restrict the intergranular oxidation to 3 μm or less from the surfaceby adjusting the oxidation inhibitive atmosphere, the heatingtemperature, the heating time, etc., at the time of carburization.

The intergranular strength decreases if an intergranular oxide (portion)is produced. Therefore, if intergranular oxidation reaches a depthbeyond 3 μm, there is a danger of reduced abrasion resistance due toinsufficient strength of the member, reduced hardness, etc. Furthermore,at the time of intergranular oxidation, surrounding alloy elements arealso taken up into the intergranular oxide due to chemical reactions.Therefore, the hardenability-improving elements in the carburized andhardened layer around intergranular oxides are taken up and consumed bythe intergranular oxides, thereby forming regions where additives aredepleted, around the intergranular oxide layer. Therefore, thehardenability of the carburized and hardened layer becomes insufficient.Hence; there is a danger of causing insufficient hardness andinsufficient strength.

It is also preferable that the raw material have a surface compressionresidual stress of 300 to 800 MPa. That is, it is preferable to set thesurface compression residual stress to at least 300 MPa by adjusting thecomposition of the raw material, the oxidation inhibitive atmosphere forthe carburization, the heating temperature, the heating time, etc.Therefore, the tensile stress near the surface can be reduced by thecompression residual stress near the surface of the member. Inparticular, the dynamic strength (planer fatigue strength, bendingfatigue strength, torsional fatigue strength) can be improved. If thesurface compression residual stress is greater than 800 MPa, it isnecessary to increase the cooling rate during the quenching processbeyond a limit in order to increase the amount of martensite. Therefore,great hardening strain occurs, and therefore a dimensional accuracy ofthe member cannot be secured.

The surface compression residual stress can be produced by forming themartensite via the quenching process of the carburized layer, andcreating a compression stress field due to volume expansion involved inthe transformation. However, if the amount of martensite produced issmall, that is, if the amount of retained austenite is great, or if thetroostite structure is great in amount, it is impossible to form asufficient compression residual stress field. Therefore, the reductionof the retained austenite (specifically, to 25% or less) and thereduction of the troostite structure (specifically, to 10% or less) areeffective in view of enhancement of compression residual stress effect.The absorption of volume expansion at the time of martensitetransformation does not considerably contribute to enhancement of thesurface compression residual stress if the amount of martensite issmall. If the amount of martensite is small, plastic deformation of thesurrounding retained austenite or troostite structure is involved, andtherefore stress reduces. However, if the amount of martensite increasesand the retained austenite or troostite structure reduces in amount asmentioned above, the density of dislocation introduced by plasticdeformation increases, so that slip is restrained. Therefore, thesurface compression residual stress rapidly increases.

In another possible method, the compression residual stress can beincreased by performing a surface process, such as shot peening, afterthe quenching process. In the latter method, turning the retainedaustenite into martensite by the shot peening process is moreadvantageous in increasing the compression residual stress.

It is also preferable that in the quenching process, quenching beperformed with the severity of quenching H being in said range during atransition from a temperature in an austenite region to 300° C.Therefore, sufficient quenching effect can be achieved. If the severityof quenching H in a cooling process from the temperature of theaustenite region to 300° C. is less than 0.01 (cm⁻¹), the quenching willbe insufficient. Thus, desired hardened structure and characteristiccannot be achieved, and the strength of the member will be insufficient.If the severity of quenching H in a cooling process from the temperatureof the austenite region to 300° C. is greater than 0.08 (cm⁻¹), thequenching will be excessive, so that the structure transformation stressand the thermal stress will increase. Therefore, there is a possibilityof increased hardening strain and degraded component part accuracy.

It is also preferable that in the quenching process, quenching beaccomplished by gas cooling. Therefore, it becomes relatively easy tosecure the aforementioned severity of quenching H.

It is also preferable that the quenching by gas cooling use an inertgas. Therefore, a safety can be secured during the quenching.

It is also preferable that the inert gas be a nitrogen gas. The adoptionof nitrogen gas as the aforementioned inert gas is preferable in view ofcost, ease of handling, availability at the time of mass-productionoperation, etc.

In the carburized and hardened member of the second aspect of thepresent invention, a retained austenite area rate of the carburizedlayer preferably is at most 25%. If the retained austenite area rate isgreater than 25%, structural transformation from retained austenite intomartensite occurs in association with changes in temperature andoperating stress during a working process after the carburizing andquenching process, or during the use of the member. Due to the stress ofthe transformation, strain occurs, and the component parts accuracy willlikely degrade. It is more preferable that the retained austenite arearate be 20% or less. The retained austenite area rate can be reduced byother manners. For example, the area rate can be reduced by forciblyturning the retained austenite into martensite via shot peening or thelike.

It is also preferable that a troostite structure area rate of a surfacelayer of the carburized layer be at most 10%. The troostite is aslack-quenched structure formed in the carburized layer after thecarburizing and quenching process, and has a low hardness. Therefore, ifthe troostite structure area rate is greater than 10%, low-strengthtroostite will reduce the strength of the component part.

It is also preferable that an internal structure of the carburized andhardened member be bainite. More-specifically, it is desirable that thearea rate of bainite in a sectional structure be at least 50%. Unlikethe case of martensite, transformation of bainite progresses while ironatoms forming a lattice partially diffuse. Therefore, the strainassociated with transformation is less in bainite than in martensite.Furthermore, bainite has a greater hardness than pearlite, which isproduced if the cooling rate is lower. Thus, bainite appropriatelyenhances the strength of the internal non-carburized layer. In order toform an internal layer portion mainly from bainite, it is desirable toselect such a composition as to form a structure mainly from bainite bysetting the severity of quenching H within the range of 0.01 to 0.08(cm⁻¹). Therefore, it becomes possible to provide a component part thathas high strength and high toughness.

It is also preferable that the carburized and hardened member be acarburized toothed gear. The toothed gears require various strictconditions. The excellent characteristics achieved by theabove-described production method are very effective for the toothedgears.

EXAMPLES

The carburized and hardened members according to embodiments of thepresent invention will be described in detail with reference to specificexamples.

Example 1

As Example 1, results of experiments conducted to verify advantages ofthe present invention will be described.

Steels (Steel 11 to Steel 14) having chemical compositions shown inTable 1, after being melt-formed in an arc furnace, were hot-rolled intoround bars having a diameter of 150 mm and a diameter of 32 mm. Theround bars were normalized by keeping them at 925° C. for an hour andthen air-cooling them.

Steel 11 and Steel 12 are steel grades having new compositions developedin the example. Steel 13 and Steel 14 are steel grades corresponding tocase hardening steels SCM420 and SNCM 815 according to JIS.

Firstly, for each steel grade, a hardenability J was determined byconducting a Jominy end quenching method according to JIS: G0561.

Results are shown in Table 1. This characteristic is a characteristic ofa raw material irrelevant to the production method described below.TABLE 1 Steel Component element (wt %) grade C Si Mn S Ni Cr Mo B Ti MbAl N Hardenability J 11 0.16 0.56 0.38 0.012 0.96 1.47 0.01 0.0022 0.0440.05 0.013 0.006 12 0.18 0.75 0.35 0.009 0.71 2.22 0.01 0.0018 0.0350.03 0.019 0.005 42 13 0.20 0.21 0.78 0.011 0.02 1.01 0.17 — — — 0.0270.015 25 14 0.15 0.25 0.47 0.009 4.34 0.83 0.27 — — — 0.040 0.018 37

As can be understood from Table 1, Steels 11 and 12 are alloy steelsthat are applicable as a raw material in the present invention in viewof material quality and hardenability J. However, as for Steel 13, thehardenability J and the Si content are outside their respective rangesaccording to the present invention. As for Steel 14, the Si content isoutside the range according the present invention.

Steels 11 to 14 were formed into round bar test pieces (not shown) of 25mm in diameter and 50 mm in length, and were also formed into rotatingbending fatigue test pieces 1 having a shape as shown in FIG. 1.

Normalized materials of 150 mm in diameter were machined into test spurgears 4 having a pitch radius of 54 mm, 27 teeth, a module of 4, afacewidth of 9 mm, a shaft hole radius of 35 mm (an equivalent round bardiameter of 10.5 mmφ) as shown in FIG. 2.

The test pieces and the gears produced from Steels 11, 12 and 14 weresubjected to low-pressure carburization (vacuum carburization) and gasquenching under the conditions of “Process 1” shown in Table 2.

The test pieces produced from Steel 13 were gas-carburized andoil-quenched under the conditions of “Process 2” shown in Table 3.

In the aforementioned “Process 1”, the severity of quenching H after thecarburization is 0.05 (cm⁻¹) as shown in Table 2, and the elements ofthe production method of the present invention are included.

In the aforementioned “Process 2”, the severity of quenching H after thecarburization is 0.15 (cm⁻¹) as shown in Table 3, and the elements ofthe production method of the present invention are included.

The test pieces prepared as described above were subjected to thefollowing tests.

First, with regard to the round bar test pieces of 25 mm in diameter, ahardness distribution (internal hardness) of a cross section wasinvestigated using a Vickers hardness meter. The surface layer hardness(surface hardness) of each carburized member was measured at a positionof 0.02 mm from the surface. Furthermore, at an equivalent position, thetroostite area rate was measured by image analysis of scanning electronmicrographs.

As for the intergranular oxidation layer, a greatest depth of theoxidation layer from the superficial metallographic structure wasmeasured by an optical microscope.

The surface carbon concentration was measured at a position of 50 μmfrom the surface via an X-ray macroanalyzer.

The retained austenite area rate was measured at a surface of the memberusing a Co—Kα ray in an X-ray diffraction apparatus.

The surface residual stress was measured by a half value breadthmidpoint method, using an Fe—Kα ray in an X-ray stress meter.

Measurement results are shown in Table 4. TABLE 2 Process 1 Tem-Severity of Step perature Time Atmosphere Pressure Quenching HCarburizing 930° C.   2 h Acetylene 20 mbar — Diffusion 930° C.   1 hAcetylene 20 mbar — Thermal 850° C. 0.5 h Acetylene 20 mbar — uniformingQuenching — — Nitrogen  8 bar 0.05 cm⁻¹ Tempering 150° C.   2 hAtmosphere Atmospheric —

TABLE 3 Process 2 Severity of Step Temperature Time Atmosphere PressureQuenching H Carburizing 930° C.   3 h Mixed gas of CO, H₂, N₂,Atmospheric — etc. formed by reaction of butane and air Diffusion 930°C.   1 h Mixed gas of CO, H₂, N₂, Atmospheric — etc. formed by reactionof butane and air Thermal 850° C. 0.5 h Mixed gas of CO, H₂, N₂,Atmospheric — uniforming etc. formed by reaction of butane and airQuenching 120° C. — Oil Atmospheric 0.15 cm⁻¹ Tempering 150° C.   2 hAtmosphere Atmospheric —

TABLE 4 Retained Surface 10⁷ fatigue limit Intergranular Surface carbonTroostite Surface austenite residual internal Bending Plane SteelCarburizing and oxidation layer concentration area rate hardness arearate stress hardness fatigue fatigue grade quenching step (μm) (%) (%)(%) (%) (MPa) (Hv) (MPa) (MPa) 11 (Process 1) 1.2 0.68 7.0 779 14.2 −314393 1098 3750 12 vacuum 2.2 1.21 2.5 839 19.1 −330 423 1080 4260carburizing + gas cooling 13 (Process 2) 10.7 0.78 37.7 631 7.1 −69 267900 3000 gas carburizing + oil cooling 14 (Process 1) 5.8 0.66 9.1 72922.5 −125 384 1053 3090 vacuum carburizing + gas cooling

As shown in Table 4, all the carburized and hardened specimens “Steel11, 12+Process 1” produced from Steels 11 and 12 by Process 1(hereinafter, combinations of the steel grade and the production processwill be indicated in the fashion of “Steel Grade+Process”) had a centralportion hardness above 250 Hv. The structures in a surface layer and acentral portion were martensite, and no remarkable slack-quenchedstructure existed.

In contrast, the specimen “Steel 13+Process 2” had a lower surface layerhardness and a lower central portion hardness than any one of thespecimens “Steel 11, 12+Process 1”.

The specimen “Steel 14+Process 1” had a surface layer hardness and acentral portion hardness that are approximately equal to those of thespecimens “Steel 11, 12+Process 1”, but had a greater retained austenitearea rate and a smaller surface residual stress. Correspondingly, themember was inferior in the plane fatigue strength.

As for the rotating bending fatigue test, an Ono-type rotary bendingfatigue testing machine was used to determine fatigue strengths with thereference number of repetitions being ten millions. Results are shown asthe bending fatigue and the plane fatigue in Table 4.

As can be understood from Table 4, the specimens “Steel 11, 12+Process1” achieved considerably better characteristics in the rotating bendingfatigue strength than the specimens “Steel 13+Process 2” and “Steel14+Process 1”.

As for the gears, the gear accuracy and the dimensional accuracy wereevaluated as described below.

To evaluate the gear accuracy, an amount of error in directions of gearpressure and an amount of error in the direction of helix angle weremeasured on each of the right and left tooth flanks, via a dedicatedprecision gear accuracy measuring machine. Tooth space heights weremeasured all round the circumference of each gear, and a value obtainedby subtracting a minimum value from a maximum value was determined as atooth space runout.

To evaluate the dimensional accuracy, a ball was placed in two toothspaces of gears facing each other, and an outer periphery thereof wasmeasured via a dedicated OBD measuring device. As for the OBDmeasurement, circumferential directions were two perpendiculardirections (X, Y), and upper, intermediate and lower sites (three sites)(A, B, C) were defined in the direction of facewidth, as indicated inFIGS. 2 a and 2 b. As an OBD ellipse, an absolute value of thedifference in OBD in the two perpendicular directions was determined. Asan OBD taper, a difference between an upper OBD and a lower OBD in thedirection of facewidth was determined.

Results are shown in FIG. 5. TABLE 5 Gear accuracy (%) Carbu- Variationin Dimensional rizing characteristics accuracy (%) and Pressure HelixTooth OBD OBD Steel quenching Tooth angle angle space vari- el- OBDgrade step flank error error runout ation lipse taper 11 Process 1 Right45 51 68 70 82 35 Left 48 49 12 Process 1 Right 62 65 73 78 81 40 Left58 60 13 Process 2 Right 100 100 100 100 100 100 Left 100 100 14 Process1 Right 47 48 70 65 80 30 Left 50 55

As can be understood from Table 5, the specimens “Steel 11, 12+Process1” exhibited better gear accuracies and better dimensional accuraciesthan the other members.

The aforementioned results indicate that it is possible to increase thestrength while sufficiently reducing the hardening strain in thespecimens “Steel 11, 12+Process 1” in which a specific alloy steelhaving a C content, an Si content and hardenability J within theaforementioned specific ranges was used as a raw material, and wassubjected to a carburizing process in an oxidation inhibitiveatmosphere, thereby forming a carburized layer, and then the steel wasquenched under the condition of the specific severity of quenching H.

As for the alloy steel, it is appropriate to make a setting such thatthe alloy steel contains Fe as a main component and, as subsidiarycomponents, 0.12 to 0.22 wt. % of C, 0.5 to 1.5 wt. % of Si, 0.25 to0.45 wt. % of Mn, 0.5 to 1.5 wt. % of Ni, 1.3 to 2.3 wt. % of Cr, 0.001to 0.003 wt. % of B, 0.02 to 0.06 wt. % of Ti, 0.02 to 0.12 wt. % of Nb,and 0.005 to 0.05 wt. % of Al.

More specifically, it is appropriate to prepare a composition such thata component parameter N defined as below is 95 or less.N≡106×C(wt. %)+10.8×Si(wt. %)+19.9×Mn(wt. %)+16.7×Ni(wt. %)+8.55×Cr(wt.%)+45.5×Mo(wt. %)+28

In Steel Grades 11, 12, N is 87.6 and 93.4, respectively, whereas inSteel Grades 13, 14, not included in the present invention in terms ofthe ranges of components, N is greater than 95. If N is greater than 95,the hardness of the steel in the rolled state or the hardness of thesteel in the normalized state remarkably increases, so that neitherrequired machine workability nor required cold workability can beachieved. Therefore, if productivity is highly valued, it is necessaryto control the composition of the steel so that the component parameterN is less than or equal to 95.

In the alloy steel satisfying the component ranges according to thepresent invention, no bainite is produced if the cooling rate is equalto or less than 0.1° C./sec., and no ferrite is produced if the coolingrate is greater than or equal to 12° C./sec. These ranges of the coolingrate can be specified through measurements of continuous coolingtransformation diagrams (CCT diagrams) of a steel at various coolingrates.

In the present invention, the composition of the steel is set so that noferrite is produced in a range of cooling rate greater than or equal to12° C./sec. (hereinafter, referred to as “upper limit cooling rate), inorder to ensure that the sufficient hardening of the carburized layercan be achieved even by gas cooling. If ferrite is produced although thecooling rate is greater than or equal to 12° C./sec., it is impossibleto accomplish the sufficient production of martensite in the carburizedlayer by gas cooling, leading to insufficient hardness.

However, excessively high hardenability is disadvantageous, too. Thatis, if martensite is excessively produced in the internal layer portionwhere the carburization does not have effect, the production ofmartensite in the entire member becomes considerably great, leading todegraded dimensional accuracy. Therefore, it is important to select acomposition so that at the time of gas quenching, martensite issufficiently produced in the carburized layer whereas martensite is notexcessively produced in the internal layer portion. Specifically, thecomposition of the steel is set so that if the cooling rate is less thanor equal to 0.1° C./sec., no bainite is produced. If bainite is producedeven though the cooling rate is less than or equal to 0.1° C./sec., thehardening reaches the internal layer portion, which is not affected bythe carburized layer. Thus, strain increases.

If the setting is made so that no bainite is produced if the coolingrate is less than 0.1° C./sec., production of bainite its sufficientlyprevented or reduced in an actual range of annealing cooling rate, sothat a highly workable structure with a large amount of ferrite andpearlite can be provided. Therefore, if the rate of cooling fromaustenite is within a range corresponding to the annealing state, thatis, a state where the material is air-cooled or let stand to cool, thematerial is provided with a hardness that is sufficiently low to improvethe workability. Thus, the working prior to the carburizing andquenching process becomes easier.

Furthermore, it is desirable to select such a composition that aninternal layer portion can be provided with a structure in which bainiteis major if the cooling rate is set at 0.1 to 1° C./sec. It isparticularly desirable to select such a composition that the cooling at3° C./sec. will provide a structure mainly formed by bainite.

Example 2

In this example, steels indicated in Table 6 (Steels 21 to 24 and Steels31 to 38) were melted and formed into ingots, which were bloom-rolledand bar-rolled to produce round bars of 70 mm in diameter.

Subsequently, the round bars of 70 mmφ were stretched to 120 mmφ by hotforging. After being normalized at 925° C., the materials were formedinto test pieces and toothed gears as in Example 1 (see FIGS. 1 and 2).

The test pieces and the gears were processed separately by threedifferent production methods (Processes 3 to 5).

“Process 3” is characterized by gas carburization and oil quenching. Inthis process, steel is carburized and quenched and then tempered in acarburizing gas atmosphere in the manner of heating at 930° C. for 5hours→diffusion at 850° C. for 1 hour→oil-quenching at 130° C.→temperingat 180° C. for 1 hour. The severity of quenching H in this case is 0.15(cm⁻¹).

“Process 4” is characterized by vacuum carburization and gas cooling. Inthis process, steel is carburized and quenched and then tempered in themanner of heating at 930° C. for 5 hours→diffusion at 850° C. for 1hour→nitrogen gas cooling→tempering at 180° C. for 1 hour. The severityof quenching H in this case is 0.05 (cm⁻¹).

“Process 5” is similar to Process 4, except that the nitrogen gascooling in Process 4 is changed to oil quenching at 130° C. The severityof quenching H in this case is 0.15 (cm⁻¹).

The test pieces and the gears processed by the above-described processwere subjected to measurements, tests, and the like as in Example 1.

Results are shown in Tables 7 and 8.

As shown in Tables 7 and 8, Steel Grades 31 to 38 were inferior in thebending fatigue strength and the plane fatigue strength; furthermore,the oil-cooled component parts had great variation in precision due tohardening strain, and therefore would have problems in practical use.

Steel Grades 31 to 34 had a slack quenched structure due tointergranular oxidation formation at the time of gas carburization, andtherefore exhibited low surface hardness and low strengths. Furthermore,since oil cooling causes rapider quenching and greater non-uniformity incooling than gas cooling, the variation in precision due to hardeningstrain increased.

In Steel Grades 37, 38, the quenching by oil-cooling was excessivelystrong with respect to the hardenability of the steel materials, so thatthe internal hardness excessively increased. The difference between theproportion of the surface structure transformation and the proportion ofthe internal structure transformation was relatively small, that is, thedifference between the surface hardness and the internal hardness wasrelatively small. Therefore, the surface layer residual stress wasrelatively small, and the strengths were relatively low. Furthermore,since oil cooling causes rapider quenching and greater coolingnon-uniformity than gas cooling, the variation in precision due tohardening strain increased.

In contrast, each of Steel Grades 21 to 24 exhibited a high surfacehardness and an appropriate value of internal hardness, and reducedstrain. Thus, it is apparent that high strengths and low strains wereachieved.

Therefore, this example also indicates that it is possible to increasethe strength while sufficiently reducing the hardening strain in themembers if a specific alloy steel having a C content, an Si content andhardenability J within the aforementioned specific ranges is used as araw material, and is subjected to a carburizing process in an oxidationinhibitive atmosphere, thereby forming a carburized layer, and then thesteel is quenched under the condition of the specific severity ofquenching H.

As for the alloy steel, it is appropriate to make a setting such thatthe alloy steel contains Fe as a main component and, as subsidiarycomponents, 0.1 to 0.5 wt. % of C, 0.5 to 1.0 wt. % of Si, 0.3 to 1.0wt. % of Mn, 0.1 to 1.0 wt. % of Cr, 0.003 to 0.015 wt. % of P, 0.005 to0.03 wt. % of S, 0.01 to 0.06 wt. % of Al, and 0.005 to 0.03 wt. % of N,and at least one of 0.3 to 1.3 wt. % of Mo and 0.1 to 1.0 wt. % of Ni.It is also possible to contain, as subsidiary components, at least oneof 0.05 to 1.5 wt. % of V, 0.02 to 0.2 wt. % of Nb, 0.01 to 0.2 wt. % ofTi, or 0.0005 to 0.005 wt. % of B and 0.005 to 0.1 wt. % of Ti, or0.0005 to 0.005 wt. % of B and 0.11 to 0.2 wt. % of Ti. As still otherelements, at least one species selected from the group consisting of atmost 0.01% by weigh of Ca, at most 0.01% by weight of Mg, at most 0.05%by weight of Zr and at most 0.1% by weight of Te may be contained. TABLE6 Component element (wt %) Trace Steel Other & fine element grade C SiMn P S Cr Al Mo grain elements (ppm) N (ppm) Hardenability J 21 0.150.50 0.50 0.008 0.015 0.12 0.020 1.00 V: 0.10, Nb: 0.03, Te: 20, Zr: 20,142 38 Ti: 0.02 B: 20 22 0.15 0.50 0.50 0.003 0.011 0.50 0.020 0.95 Ni:0.30, V: 0.10, Ca: 20, Zr: 50, 132 44 Nb: 0.02, Ti: 0.02 B: 20 23 0.151.00 0.86 0.010 0.014 0.12 0.018 1.00 V: 0.10, Nb: 0.03, Te: 15, Zr: 10,135 38 Ti: 0.02 B: 15 24 0.25 0.50 0.35 0.015 0.012 0.90 0.035 0.91 Nb:0.02 Mg: 30 145 31 31 0.21 0.25 0.70 0.015 0.015 1.06 0.030 0.15 — — 13028 32 0.19 0.26 0.80 0.019 0.014 1.13 0.035 0.02 — — 138 26 33 0.15 0.500.50 0.010 0.015 0.44 0.035 1.01 V: 0.10, Nb: 0.03, Ca: 20 130 28 340.15 0.50 0.50 0.008 0.015 0.12 0.020 1.00 V: 0.10, Nb: 0.03, Te: 20,Zr: 20, 142 38 Ti: 0.02 B: 20 35 0.21 0.25 0.70 0.015 0.015 1.06 0.0300.15 — — 130 28 36 0.19 0.26 0.80 0.019 0.014 1.13 0.035 0.02 — — 138 2637 0.15 0.50 0.50 0.010 0.015 0.44 0.035 1.01 V: 0.10, Nb: 0.03 Ca: 20130 28 38 0.15 0.50 0.50 0.008 0.015 0.12 0.020 1.00 V: 0.10, Nb: 0.03Te: 20, Zr: 20, 142 38 Ti: 0.02 B: 20

TABLE 7 Surface 10⁷ fatigue limit Intergranular Surface carbon TroostiteSurface Retained residual Internal Bending Plane Steel Carurizing andoxidation concentration area rate hardness austenite stress Hardnessfatigue fatigue grade quenching layer (μm) (%) (%) (Hv) area rate (%)(MPa) (Hv) (MPa) (MPa) 21 (Process 4) 0 0.67 3 845 6 −392 280 1200 350022 vacuum 0 0.61 3 874 8 −370 315 1250 3500 23 carburizing + gas 0 0.683 844 7 −390 275 1200 3500 24 cooling 0 0.62 4 840 9 −390 300 1200 360031 (Process 3) 15 0.61 41 680 20 50 290 800 2800 32 gas carburizing +oil 5 0.66 28 670 22 40 280 750 2800 33 cooling 18 0.61 40 780 18 80 350900 3100 34 20 0.62 43 770 17 90 360 900 3000 35 (Process 4) 0 0.68 2813 12 −360 230 1000 3100 36 vacuum 0 0.69 3 780 13 −300 220 1000 3200carburizing + gas cooling 37 (Process 5) 0 0.66 3 780 10 −160 390 10003000 38 vacuum 0 0.64 4 850 10 −100 400 1000 3100 carburizing + oilcooling

TABLE 8 Gear accuracy (%) Variation in characteristics Helix ToothDimensional accuracy (%) Steel Carburizing and Tooth Pressure anglespace OBD OBD OBD grade quenching step flank angel error error runoutvariation ellipse taper 21 (Process 4) Right 48 60 65 55 80 36 vacuumLeft 52 54 22 carburizing + gas Right 47 55 70 68 85 48 cooling Left 4859 23 Right 60 67 66 70 77 32 Left 52 61 27 Right 51 56 64 60 79 47 Left47 52 31 (Process 3) Right 103 108 105 98 100 110 gas carburizing + oilLeft 112 105 32 cooling Right 99 105 100 100 110 105 Left 18 98 33 Right110 105 101 108 106 99 Left 105 104 34 Right 102 109 106 111 111 107Left 106 110 35 (Process 4) Right 60 59 70 65 77 43 vacuum Left 51 65 36carburizing + gas Right 59 55 78 64 85 48 cooling Left 54 59 37 (Process5) Right 99 106 105 97 110 102 vacuum Left 108 111 38 carburizing + oilRight 100 100 100 100 100 100 cooling Left 100 100

1-15. (canceled)
 16. A steel carburizing and hardening methodcomprising: providing an alloy steel, as a raw material, which containsFe as a main component, 0.10 to 0.50 wt. % C and 0.50 to 1.50 wt. % Si,said alloy steel having a hardenability J, based on an end quenchingtest, in a range of 35 to 50 at 12.5 mm; forming the alloy steel into adesired shape; carburizing the shaped alloy steel in an oxidationinhibiting atmosphere; and quenching the carburized alloy steel bycooling from a pearlite transformation point (A1 point) to a martensitetransformation start point (Ms point), with severity of quenching H in arange of 0.01 to 0.08 (cm⁻¹), and without interruption by any rise intemperature.
 17. A method according to claim 16 wherein said carburizingis performed in an atmosphere having a reduced pressure of 1 to 30 hPa.18. A method according to claim 16 wherein said carburizing is performedin an atmosphere containing an inert gas as a main component.
 19. Amethod according to claim 16 wherein said carburizing produces 0.6 to1.5 wt. % carbon in a carburized layer.
 20. A method according to claim16 wherein intergranular oxidation progresses from a surface of the rawmaterial to a depth which is at most 3 μm.
 21. A method according toclaim 16 wherein the raw material has a surface compression residualstress of 300 to 800 Mpa.
 22. A method according to claim 16 whereinsaid quenching is performed with the severity of quenching H being insaid range during transition from a temperature in an austenite regionto 300° C.
 23. A method according to claim 16 wherein said quenching isaccomplished by gas cooling.
 24. A method according to claim 23 whereinsaid quenching accomplished by the gas cooling uses an inert gas.
 25. Amethod according to claim 24 wherein the inert gas is nitrogen.
 26. Acarburized and hardened steel alloy member produced by the method ofclaim 16 having a carburized layer with a surface hardness in a range of700 to 900 Hv, and an internal non-carburized portion, located inward ofthe carburized layer, having a hardness in a range of 250 to 450 Hv. 27.The carburized and hardened steel member according to claim 26 whereinthe area of retained austenite in section of the carburized layer is atmost 25%.
 28. The carburized and hardened steel alloy member accordingto claim 26 wherein the area of troostite structure in section at thesurface of the carburized layer is at most 10%.
 29. The carburized andhardened steel alloy member according to claim 26 having an internalstructure of bainite.
 30. The carburized and hardened member accordingto claim 26 in the form of a toothed gear.